Method and apparatus for providing mutual time difference determination of base station signals in a cellular communication system

ABSTRACT

A method and an apparatus for determining mutual time difference between base station signals in an asynchronous code division multiplexing access (CDMA) system. The method comprises the steps of: measuring mutual time difference of signals transmitted between at least two base stations; determining all possible paths between said at least two base stations; and providing weights to the measured mutual time difference for said all possible paths. The apparatus comprises: a location measurement unit for measuring mutual time difference of signals transmitted between at least two base stations; a mobile user location center for receiving the mutual time difference of the signals measured by the location measurement unit, determining all possible paths between said at least two base stations, and providing weights to the measured mutual time difference for said all possible paths. The method and apparatus can determine a mutual time difference of signals transmitted from base stations even when the signals are blocked by a certain object and can improve the determination accuracy of the mutual time difference of base station signals.

PRIORITY

[0001] This application claims priority to an application entitled“Method of mutual time difference determination of base station signalsin cellular communication system” filed in the Russian IndustrialProperty Office on Nov. 15, 2002 and assigned Serial No. RU2002130594,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a radio communication system, inparticular to a method and an apparatus for providing mutual timedifference determination of base station signals in a cellularcommunication system.

[0004] 2. Description of the Related Art

[0005] Third Generation (3G) CDMA cellular base stations arenon-synchronous in for example the Third Generation Partnership Project(3GPP) Frequency Division Duplex (FDD) mode. Here, the synchronism ofbase stations (BS) refers to the synchronization of the downlinktransmission signals of different base stations.

[0006] Mutual time difference determination is required to determine themobile users' locations and to reduce time and hardware resources at theinitial base station signal search by mobile stations and to decreasethe stored data amount in the course of the soft handoff procedure. Themutual time difference determination during the soft handoff procedureis described in the Russian patent #2137314, Int. cl.6H 04 L 27/30,which is incorporated herein by reference.

[0007] One of the prior art solutions described in “Synergies BetweenSatellite Navigation and Location Services of Terrestrial MobileCommunication”, G. Hein, B. Eissfeller, V. Oehler, Jon O. Winkel,Institute of Geodesy and Navigation, University FAF Munich, ION GPS2000, 19-22 Sep. 2000, Salt Lake City, Utah, which is incorporatedherein by reference proposes the application of location measurementunits receiving BS signals and defining the BS their mutual timedifference to determine mutual time differences of base stations.

[0008] However, the disclosed solution described above has the followingproblems:

[0009] First, non-line of sight multipath signal propagation from basestations to location measurement units causes low accuracy whendetermining mutual time differences of base station signals.

[0010] Second, it is impossible to get the mutual time difference of thebase station signals when the direct measurement is unobtainable.

[0011] The prior art solution to overcome such problems is the solutiondescribed in 3GPP Technical Specification (TS) 25.305 V3.7.0, 2001 12,“Stage 2 Functional Specification of UE Positioning in UTRAN”, which isincorporated herein by reference.

[0012] This solution proposes to receive base station signals, fromwhich the mutual time difference is determined by the locationmeasurement unit which is located in a position with known coordinates.FIG. 1 helps in understanding the method of mutual time differencedetermination of base station signals in a cellular communication systemperformed by 3GPP TS 25.305 V3.7.0, 2001 12.

[0013]FIG. 1 shows base stations 1 and 2, the mutual time difference ofwhich is to be determined, a location measurement unit (LMU) 3, basestation controller (BSC) 4 and a mobile user location center (MULC) 5.

[0014] The LMU 3 receives a signal of the base station (BS) 1 and asignal of the BS 2 and executes a specified number of sequentialmeasurements of the signals' time difference. Further, the LMU 3averages the sequential measurements of time difference of BS 1 and BS 2signals, thus obtaining the averaged measured time difference of thesignals of the given base stations.

[0015] The LMU 3 determines the accuracy of the averaged measured timedifference, for example, by the signal-to-noise ratio of the BS 1 and BS2. In this case a value linearly connected with an error of the averagedmeasured time difference to the true value of the time difference of theBS 1 and 2 signals received by the location measurement unit 3 isselected as the averaged measured time difference accuracy.

[0016] The averaged measured time difference of the BS 1 and BS 2signals and its accuracy are transmitted from the LMU 3 to the BS 1 orBS 2 using the current radio interface and then transmitted from therespective BS to the base station controller 4 by a wire communicationline connecting the BSC 4 and the base station.

[0017] The BSC 4 determines the mutual time difference of the BS 1 andBS 2 signals by the averaged measured time difference of the BS 1 and BS2 signals by considering the known mutual location of the BS 1 and BS 2and the LMU 3.

[0018] The obtained mutual time difference of the BS 1 and BS 2 signalsand its accuracy are transmitted from the BSC 4 to the mobile userlocation center 5 for further application.

[0019] Thus, the following main features of implementing the method canbe emphasized from the description of the prior art method of mutualtime difference determination of base station signals in a cellularcommunication system:

[0020] In each location, a measurement unit performs the sequentialmeasurement of mutual time difference of signals of at least two basestations, signals of which are received in the location measurementunit, averaging of these time difference measurements results in theaveraged measured mutual time difference of these base stations, anddetermines its accuracy.

[0021] The averaged measured time differences and their accuracies aretransmitted from each location measurement unit to one of the basestations which signals are received by the location measurement unit,and then to the base station controller that controls the base station.

[0022] The mutual time difference of signals is determined for each basestation pair for the base station controller using the averaged measuredtime difference of the given base station signals.

[0023] However, the known method has a number of importantdisadvantages.

[0024] First, determination of the mutual time difference of the basestation signals by the averaged measured time difference of these basestation signals may be insignificantly accurate. The inaccuracy iscaused by the fact that the estimate of the mutual time difference ofthe base station signals is subject to the impact of noise errors,intra-system interference and multipath errors.

[0025] The difference between the delays of line of sight base stationsignals propagation to the location measurement unit can be removed withthe use of the known coordinates of the base stations and locationmeasurement unit.

[0026] When Δt_(1−>2) is marked as an estimate of the mutual timedifference of signals of the base station pair, especially the signalsof the first BS to the signal of the second BS, and Δ{tilde over(t)}_(1−>2) is marked as the true value of the mutual time difference ofsignals of the base stations, the difference between the estimateΔt_(1−>2) and the true value Δ{tilde over (t)}_(1−>2) can be expressedby the following Equation 1.

Δt _(1−>2) −Δ{tilde over (t)}_(1−>2)=ε_(noise)+ε_(multipath,1)−ε_(multipath,2)  Equation 1

[0027] In equation 1, ε_(noise) is an error defined by noise andintra-system interference, ε_(multipath,1) is a multipath error of thefirst BS, equal to the difference between the real propagation time ofthe first BS signal to the location measurement unit and the knownpropagation time of the first BS signal to the location measurementunit, and ε_(multipath,2) is a multipath error of the second BS, equalto the difference between the real propagation time of the second BSsignal to the location measurement unit and the known propagation timeof the second BS signal to the location measurement unit.

[0028] The averaging of the estimate Δt_(1−>2) of the mutual timedifference of signals of the first BS and second BS in the locationmeasurement unit results in the reduction of the noise error valueε_(noise). The multipath error differenceε_(multipath,1)-ε_(multipath,2) is invariable since it is defined by themutual location of the first and second BS and the location measurementunit and the surrounding scattering objects (buildings, mountains,hills, etc.) as well.

[0029] Hence, the accuracy of the claimed method of mutual timedifference determination of base station signals in a cellularcommunication system may be insufficient for a location.

[0030] Second, there may be a situation when there is no direct timesignal difference measurement between any base stations, even when theirmutual time difference is required.

[0031]FIG. 2 illustrates the above situation, demonstrating the basestations 6, 7 and 8, location measurement units 9 and 10 and a building11. Each of the BS 6, BS 7 and BS 8 transmits the first and secondsignals which comprise a group signal.

[0032] The base station group signal refers to a signal transmitted fromthe base station and having the Synchronization CHannel (SCH), CommonPIlot Channel (CPICH) and Primary Common Control Physical CHannel(P-CCPCH) and other channels as well.

[0033] Each of the LMU 9 and the LMU 10 receives signals transmittedfrom surrounding base stations and measures the signals time difference.

[0034] Referring to FIG. 2, the LMU 9 receives the first signals of theBS 6 and BS 7 and measures their time difference. The LMU 10 receivesthe second signal of the BS 7 and the first signal of the BS 8 andmeasures their time difference.

[0035] However, the second signal of the BS 6 is held by the building 11and cannot be received by the LMU 10. The second signal of the BS 8 isalso held by the building 11 and cannot be received by the LMU 9.

[0036] When the direct measurement of the time difference of the signalsof the BS 6 and BS 8 cannot be performed as described above, theconventional methods do not propose any solution capable of determiningthe mutual time difference between the signals of the BS 6 and BS 8.

SUMMARY OF THE INVENTION

[0037] Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and an object ofthe present invention is to provide a method and an apparatus which candetermine a mutual time difference of signals transmitted from basestations even when the signals are blocked by a certain object.

[0038] It is another object of the present invention is to provide amethod and an apparatus which can improve the determination accuracy ofthe mutual time difference of base station signals.

[0039] In order to substantially accomplish this object, there isprovided a method for determining mutual time difference between basestation signals in an asynchronous code division multiplexing access(CDMA) system, the method comprising the steps of: measuring mutual timedifference of signals transmitted between at least two base stations;determining all possible paths between said at least two base stations;and providing weights to the measured mutual time difference for saidall possible paths.

[0040] In accordance with another aspect of the present invention, thereis provided an apparatus for determining mutual time difference betweenbase station signals in an asynchronous code division multiplexingaccess (CDMA) system, the apparatus comprising: a location measurementunit for measuring mutual time difference of signals transmitted betweenat least two base stations; a mobile user location center for receivingthe mutual time difference of the signals measured by the locationmeasurement unit, determining all possible paths between said at leasttwo base stations, and providing weights to the measured mutual timedifference for said all possible paths.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The above and other objects, features and advantages of thepresent invention will be more apparent from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

[0042]FIG. 1 is a block diagram illustrating an example of a positioningsystem according to a Third Generation Partnership Project TechnicalSpecification (3GPP TS) 25.305 V3.7.0, 2001 12, “Stage 2 FunctionalSpecification of User Equipment (UE) Positioning in Universal MobileTelecommunications System Terrestrial Radio Access Network (UTRAN)”;

[0043]FIG. 2 is a block diagram illustrating an example of anarrangement of base stations, location measurement units and buildingswhen there is no direct measurement of the time difference of basestation signals;

[0044]FIG. 3 is a block diagram illustrating an example of a radiocommunication system in accordance with an embodiment of the presentinvention;

[0045]FIG. 4 is a block diagram illustrating an example of a locationmeasurement unit for determining the averaged measured time differenceof base station signals in accordance with an embodiment of the presentinvention;

[0046]FIG. 5 is a block diagram illustrating an example of a radiocommunication system having base stations as the vertexes and theadjusted time differences of the base station signals as the arcs inaccordance with an embodiment of the present invention;

[0047]FIG. 6 is a block diagram illustrating an example of the formingof paths from the first base station to the second base station fordetermining mutual time difference of signals in accordance with anembodiment of the present invention;

[0048]FIG. 7 is a flow diagram illustrating an example of a base stationin accordance with an embodiment of the present invention;

[0049]FIG. 8 is a flow diagram illustrating an example of the basestation operation in accordance with an embodiment of the presentinvention;

[0050]FIG. 9 is a flow diagram illustrating an example of a method ofoperation for the base station controller in accordance with anembodiment of the present invention;

[0051]FIG. 10 is a flow diagram illustrating another example of a methodof operation for the base station controller in accordance with anembodiment of the present invention;

[0052]FIG. 11 is a flow diagram illustrating an example of a method ofoperation for the mobile user location center in accordance with anembodiment of the present invention;

[0053]FIG. 12 is a flow diagram illustrating an example of a method forperforming a mobile user location procedure in accordance with anembodiment of the present invention; and

[0054]FIG. 13 is a flow diagram illustrating an example of a method forperforming accurate base station location determination in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0055] Embodiments of the present invention will be described withreference to the accompanying drawings. In the following description ofthe present invention, a detailed description of known functions andconfigurations incorporated herein will be omitted for conciseness.

[0056] In acquiring more precise time difference information betweenbase stations for measurement of locations in an asynchronous mobilecommunication system, in which no synchronization is made between themobile stations, the embodiment of the present invention provides amethod that is capable of utilizing mutual time difference informationbetween various base stations, so as to reduce error in the timedifference information which may be caused when a first and second basestations forming a pair among the various base stations are located in adowntown area in which propagation delay may frequently happen. Itshould be noted that the embodiment of the present is not limited tobase stations in downtown areas, the base stations can be used in anenvironment where signals can be blocked or affected by physicalobjects.

[0057] In order to help in understanding the embodiment of the presentinvention, an example of the method of mutual time differencedetermination of base station signals in a cellular communication systemproposed by the present invention will be described with reference toFIGS. 3 to 6.

[0058]FIG. 3 is a block diagram illustrating an example of a radiocommunication system in accordance with an embodiment of the presentinvention.

[0059] Referring to FIG. 3, the cellular radio communication systemincludes a plurality of base stations (BSs) 12, 13, 14, 15 and 16,location measurement units (LMUs) 17, 18, 19 and 20 for receiving atleast two base station signals from the BSs 12, 13, 14, 15 and 16through a radio interface, a base station controller (BSC) 21 connectedthrough wire communication lines to the BSs 12, 13, 14, 15 and 16 tocontrol the BSs 12, 13, 14, 15 and 16, and a mobile user location center(MULC) 22 connected to the BSC 21 through a wire communication line. The(MULC)₂₂ could be embedded in the BSC 21.

[0060] The BS 12 transmits a signal being its group signal. The BS 13transmits the first and second signals being its group signal. The BS 14transmits the first and second signals being its group signal. The BS 15transmits the first and second signals being its group signal. The BS 16transmits the first and second signals being its group signal.

[0061] The base station group signal refers to a signal transmitted fromthe base station comprising the following channels: SynchronizationCHannel (SCH), Common PIlot Channel (CPICH), Primary Common ControlPhysical CHannel (P-CCPCH) and other channels as well.

[0062] The LMU 17 receives the signal of the BS 12 and the first signalof the BS 13, and executes sequential measurements of the timedifference of the first signal of the BS 13 relating to the signal ofthe BS 12. These time difference measurements are averaged, thusobtaining the averaged measured time difference Δt_(13−>12,17) of thefirst signal of the BS 13 regarding the BS 12 signal. Its accuracyσ_(13−>12,17) is determined.

[0063] The LMU 17 also receives the first signal of the BS 13 and thefirst signal of the BS 14, and executes sequential measurements of thetime difference of the first signal of the BS 13 relating to the signalof the BS 14. These time difference measurements are averaged, thusobtaining the averaged measured time difference Δt_(13−>14,17) of thefirst signal of the BS 13 with regard to the first signal of the BS 14.Its accuracy σ_(13−>14,17) is determined.

[0064] The LMU 17 also receives the first signal of the BS 12 and thefirst signal of the BS 14, and performs sequential measurements of thetime difference of the BS 12 signal with respect to the first signal ofthe BS 14. These time difference measurements are averaged, thusobtaining the averaged measured time difference Δt_(12−>14,17) of the BS12 signal with respect to the first signal of the BS 14. Its accuracyσ_(12−>14,17) is determined.

[0065] The LMU 20 receives the first signals of the BS 15 and BS 16, andperforms sequential measurements of the time difference of the firstsignal of the BS 16 with respect to the first signal of the BS 15. Thesetime difference measurements are averaged, thus obtaining the averagedmeasured time difference Δt_(16−>15,20) of the first signal of the BS 16with respect to the first signal of the BS 15. Its accuracyσ_(16−>15,20) is determined.

[0066] The LMU 18 receives the second signals of the BS 13 and BS 15,executes sequential measurements of the time difference of the secondsignal of the BS 15 with respect to the second signal of the BS 13.These time difference measurements are averaged, thus obtaining theaveraged measured time difference Δt_(15−>13,18) of the second signal ofthe BS 15 with respect to the second signal of the BS 13, and itsaccuracy σ_(15−>13,18) is determined.

[0067] The LMU 19 receives the second signals of the BS 14 and BS 16,performs sequential measurements of the time difference of the secondsignal of the BS 14 with respect to the second signal of the BS 16,averages these time difference measurements, thus obtaining the averagedmeasured time difference Δt_(14−>16,19) of the second signal of the BS14 with respect to the second signal of the BS 16. Its accuracyσ_(14−>16,19) is determined.

[0068] To illustrate, a process of obtaining the averaged timedifference Δt_(14−>16,19) of the second signal of the BS 14 with respectto the second signal of the BS 16 and determining its accuracyσ_(14−>16,19) will be explained below with reference to FIG. 4.

[0069] Herein, the radio communication system is assumed to be the 3GPPsystem in the Frequency Division Duplex (FDD) mode. Base stations of thesystem transmit signals containing SCH, CPICH and P-CCPCH.

[0070]FIG. 4 is a block diagram illustrating an example of a locationmeasurement unit for determining the averaged measured time differenceof base station signals in accordance with an embodiment of the presentinvention.

[0071] Referring to FIG. 4, the location measurement unit fordetermination of the averaged time difference of signals of the BS 14and BS 16 comprises an antenna 23, analog receiver 24, searcher 25 ofthe BS 14 group signal (hereinafter referred to as searcher 25), decoder26 of the Primary Common Control Physical Channel of the BS 14(hereinafter referred to as decoder 26), searcher 27 of the BS 16 groupsignal (hereinafter referred to as searcher 27), decoder 28 of thePrimary Common Control Physical Channel of the BS 16 (hereinafterreferred to as decoder 28) and a unit 29 of determination of theaveraged measured time difference of signals of the BS 14 and 16(hereinafter referred to as unit 29), wherein the input of the antenna23 is the input of the location measurement unit, the output of theantenna 23 is connected to the input of the analog receiver 24, theoutput of which is linked to the inputs of the searcher 25 and thesearcher 27 and to the first inputs of the decoder 26 and decoder 28,the output of the searcher 25 is connected to the first input of theunit 29 and to the second input of the decoder 26, the output of whichis connected to the second input of the unit 29, the output of thesearcher 27 is connected to the third input of the unit 29 and to thesecond input of the decoder 28, the output of which is connected to thefourth input of the unit 29, the output of which is the output of thelocation measurement unit.

[0072] In this case, the analog receiver can be embodied in the mannerprovided in the U.S. Pat. No. 5,103,459 titled “System and Method forGenerating Signal Waveforms in a CDMA Cellular Telephone System”, Int.Cl.5H 04 L 27/30, the contents of which is incorporated herein byreference.

[0073] The searcher 25 and the searcher 27 can be embodied in the mannerdescribed in 3GPP TS 25.214 V3.9.0 (2001-12), Physical layer procedures(FDD), Annex C: Cell search procedure and in “Cell Search in W-CDMA”,Yi-Pin Eric Wang and Tony Ottosson, IEEE JOURNAL ON SELECTED AREAS INCOMMUNICATIONS, VOL. 18, NO. 8, AUGUST 2000, the contents of which isincorporated herein by reference.

[0074] The decoder 26 and the decoder 28 can be embodied like thecoherent RAKE receivers of Sadayuki ABETA, Mamoru SAWAHASHI, andFumiyuki ADACHI, “Performance Comparison between Time-Miltiplexed PilotChannel and Parallel Pilot Channel for Coherent Rake Combining inDS-CDMA Mobile Radio”, IEICE Trans. Commun., Vol. E81-B, No. 7, July1998, the contents of which is incorporated herein by reference.

[0075] An input signal of the location measurement unit containing BS 14and BS 16 group signals is applied to the input of the antenna 23 and isapplied from the output of antenna 23 to the input of the analogreceiver 24. The signal is transmitted from the output of the analogreceiver 24 to the inputs of the searcher 25 and searcher 27 and to thefirst inputs of the decoder 26 and the decoder 28. The searcher 25searches the group signal of the BS 14 in the searching window using38400 chips with the Synchronization Channel and Common Pilot Channel. Asignal of the BS 14 is assumed to be acquired in the position P₁.Additionally, the searcher 25 determines the number of the primaryscrambling code of the BS 14. A signal containing the values of theacquired time position P₁ of the BS 14 signal is applied from the outputof the searcher 25 to the first input of the unit 29 and to the secondinput of the decoder 26. A signal containing the number of the primaryscrambling code of the BS 14 is applied from the output of the searcher25 to the second input of the decoder 26. With the obtained value of theacquired time position P₁ of the BS 14 signal and the obtained number ofthe primary scrambling code of the BS 14 the decoder 26 de-scrambles,demodulates and decodes the Primary Common Control Physical Channel,thus obtaining the value of the System Frame Number (SFN) of the BS 14at the moment of the transmission of the first chip of this frame. ThisSFN value of the BS 14 is labeled by SFN₁. A signal containing thedetermined value SFN₁ of the SFN of the BS 14 is applied from the outputof the decoder 26 to the second input of the unit 29.

[0076] The searcher 27 searches the group signal of the BS 16 within thesearching window using 38400 chips with the Synchronization Channel andCommon Pilot Channel. It is assumed that the BS 16 signal is acquired inthe position P₂. Additionally, the searcher 27 determines the number ofthe primary scrambling code of the BS 16. A signal containing the valueof the acquired time position P₂ of the BS 16 signal is applied from theoutput of the searcher 27 to the third input of the unit 29 and to thesecond input of the decoder 28. A signal containing the number of theprimary scrambling code of the BS 16 is applied from the output of thesearcher 27 to the second input of the decoder 28. With the obtainedvalue of the acquired time position P₂ of the BS 16 signal and theobtained number of the primary scrambling code of the BS 16 the decoder28 de-scrambles, demodulates and decodes the Primary Common ControlPhysical Channel, thus obtaining the value of the SFN of the BS 16 atthe moment of the transmission of the first chip of this frame. This SFNvalue of the BS 16 is labeled by SFN₂. A signal containing thedetermined value SFN₂ of the SFN of the BS 16 is applied from the outputof the decoder 28 to the fourth input of the unit 29.

[0077] The unit 29 defines the measured time difference Δt_(14−>16,19)of the BS 14 signal with respect to the BS 16 signal using the followingequation 2.

Δt _(14−>16,19)=(SFN ₁ −SFN ₂)T _(fr)+(P ₁ −P ₂)T _(ch)  Equation 2

[0078] In equation 2, T_(fr)—length of a frame of 3GPP base stationsignal of 10 ms, and T_(ch)—length of one chip of 1/(3.84·10⁶) s oraround 260 ns.

[0079] The unit 29 averages, for example, some sequentially measuredtime differences of the BS 14 signal with respect to the BS 16 signalobtained by equation 2, thus obtaining the averaged measured timedifference Δt_(14−>16,19) of the signal of the BS 14 with respect to thesignal of the BS 16. The unit 29 can be implemented on the DigitalSignal Processor (DSP) using the above algorithm.

[0080] If all location measurement units determine the averaged measuredtime differences of base station signals in a similar manner with thesame number of averages, the accuracies of all averaged measured timedifferences of base station signals can be defined to be the same andequal to 100 ns, for example.

[0081] In a general case with more complex and more accuratedetermination methods of the averaged measured time differences of basestation signals or with different number of averages in differentlocation measurement units, the accuracies of different averagedmeasured time differences of base station signals are different anddepend on, for example, signal-to-noise ratios of signals, whichaveraged measured time difference to be determined, and/or number ofaverages.

[0082] The averaged measured time differences Δt_(13−>12,17),Δt_(13−>14,17) and Δt_(12−>14,17) and their accuracies σ_(13−>12,17),σ_(13−>14,17) and σ_(12−>14,17) are transmitted from the LMU 17 to theBS 13 and then to the BSC 21.

[0083] The averaged measured time difference Δt_(15−>13,18) and itsaccuracy σ_(15−>13,18) are transmitted from the LMU 18 to the BS 13 andthen to the BSC 21.

[0084] The averaged measured time difference Δt_(14−>16,19) and itsaccuracy σ_(14−>16,19) are transmitted from the LMU 19 to the BS 14 andthen to the BSC 21.

[0085] The averaged measured time difference Δt_(16−>15,20) and itsaccuracy σ_(16−>15,20) are transmitted from the LMU 20 to the BS 15 andthen to the BSC 21.

[0086] As will be shown from the following equation 3, the known valueof the difference of delays at line of sight signal propagation from theBS 13 and the BS 12 to the location measurement unit 17 is subtractedfrom the averaged measured time difference Δt_(13−>12,17) in the BScontroller 21, thus obtaining the adjusted time difference Δ{tilde over(t)}_(13−>12,17) of the signal of the BS 13 with respect to the signalof the BS 12.

Δ{tilde over (t)} _(13−>12,17) =Δt _(13−>12,17)−(τ_(13−>17)−τ_(12−>17))

[0087] In equation 3, τ_(13−>17) is the signal propagation delay fromthe BS 13 to the location measurement unit 17 and τ_(12−>17) is thesignal propagation delay from the BS 12 to the location measurement unit17. The signal propagation delays can be calculated using the followingequation 4. $\begin{matrix}{{\tau_{12->17} = \frac{\sqrt{\left( {x_{12} - x_{17}} \right)^{2} + \left( {y_{12} - y_{17}} \right)^{2} + \left( {z_{12} - z_{17}} \right)^{2}}}{c}},{\tau_{13->17} = {\frac{\sqrt{\left( {x_{13} - x_{17}} \right)^{2} + \left( {y_{13} - y_{17}} \right)^{2} + \left( {z_{13} - z_{17}} \right)^{2}}}{c}.}}} & {{Equation}\quad 4}\end{matrix}$

[0088] In equation 4, c is the light speed, the BS 12 coordinates withx₁₂, y₁₂, z₁₂, the BS 13 coordinates with x₁₃, y₁₃, z₁₃, and thelocation measurement unit 17 coordinates with x₁₇, y₁₇, z₁₇. The basestations and location measurement unit coordinates can be determinedwith the use of the Global Positioning System (GPS) and/or GlobalNavigation Satellite System (GNSS) receiver.

[0089] In this case, the accuracy of the adjusted time differenceΔ{tilde over (t)}_(13−>12,17) is equal to the accuracy of the adjustedaveraged measured time difference Δt_(13−>12,17) and to σ_(13−>12,17).

[0090] The adjusted time difference Δ{tilde over (t)}_(13−>14,17) of thesignal of the BS 13 with respect to the signal of the BS 14, theadjusted time difference Δ{tilde over (t)}_(12−>14,17) of the signal ofthe BS 12 with respect to the signal of the BS 14, the adjusted timedifference Δ{tilde over (t)}_(15−>13,18) of the signal of the BS 15 withrespect to the signal of the BS 13, the adjusted time difference Δ{tildeover (t)}_(14−>16,19) of the signal of the BS 14 with respect to thesignal of the BS 16 and the adjusted time difference Δ{tilde over(t)}_(16−>15,20) of the signal of the BS 16 with respect to the signalof the BS 15 are determined in the same manner.

[0091] The adjusted time differences Δ{tilde over (t)}_(13−>12,17),Δ{tilde over (t)}_(13−>14,17), Δ{tilde over (t)}_(12−>14,17), Δ{tildeover (t)}_(15−>13,18), Δ{tilde over (t)}_(14−>16,19) and Δ{tilde over(t)}_(16−>15,20) and their accuracies σ_(13−>12,17), σ_(13−>14,17)σ_(12−>14,17), σ_(15−>13,18), σ_(14−>16,19) and σ_(16−>15,20) aretransmitted from the BSC 21 to the MULC 22.

[0092] Assume, the mutual time difference Δ{tilde over (t)}_(13−>14) ofthe signal of the BS 13 with respect to the signal of the BS 14 shouldbe determined by the MULC 22.

[0093] The true mutual time difference value of the signal of the BS 13with respect to the signal of the base station 14 is marked byΔt_(13−>14), the true mutual time difference value of the signal of theBS 13 with respect to the signal of the BS 12 is marked by Δt_(13−>12),the true mutual time difference value of the signal of the BS 12 withrespect to the signal of the BS 14 is marked by Δt_(12−>14), the truemutual time difference value of the signal of the BS 15 with respect tothe signal of the BS 13 is marked by Δt_(15−>13), the true mutual timedifference value of the signal of the BS 14 with respect to the signalof the BS 16 is marked by Δt_(14−>16) and the true mutual timedifference value of the signal of the BS 16 with respect to the signalof the BS 15 is marked by Δt_(16−>15).

[0094] Since Δt_(13−>14)=Δt_(13−>12)+Δt_(12−>14) andΔt_(13−>12)=−Δt_(15−>13)−Δt_(16−>15)−Δt_(14−>16), the mutual timedifference Δ{tilde over (t)}_(13−>14) can be estimated by three methods:

Δ{tilde over (t)} _(13−>14,17)

Δ{tilde over (t)} _(13−>12,17) +Δ{tilde over (t)} _(12−>14,17) H

−Δ{tilde over (t)} _(15−>13,18) −Δ{tilde over (t)} _(16−>15,20) −Δ{tildeover (t)} _(14−>16,19).

[0095] The configuration composed of the BS 12 through BS 16 and theadjusted time differences Δ{tilde over (t)}_(13−>12,17), Δ{tilde over(t)}_(13−>14,17), Δ{tilde over (t)}_(12−>14,17), Δ{tilde over(t)}_(15−>13,18), Δ{tilde over (t)}_(14−>16,19) and Δ{tilde over(t)}_(16−>15,20) is presented in the form of a graph depicted in FIG. 5.

[0096] Referring to FIG. 5, the vertexes of the graph are the BS 12through BS 16 and its arcs are the adjusted time differences Δ{tildeover (t)}_(13−>12,17), Δ{tilde over (t)}_(13−>14,17), Δ{tilde over(t)}_(12−>14,17), Δ{tilde over (t)}_(15−>13,18), Δ{tilde over(t)}_(14−>16,19) and Δ{tilde over (t)}_(16−>15,20).

[0097] The graph arc directions complying with the directions of theadjusted time difference are set. For example, the arc between the BS 13and BS 14 matches the adjusted time difference Δ{tilde over(t)}_(13−>14,17) of the signal of the BS 13 with respect to the signalof the BS 14 and directs from the BS 13 to the BS 14, respectively.

[0098] Tree estimates Δt_(13−>14,17), Δt_(13−>12,17)+Δt_(12−>14,17) and−Δt_(15−>13,18)−Δt_(16−>15,20)−Δt_(14−>16,19) of the mutual timedifference Δ{tilde over (t)}_(13−>14) correspond to three paths of thegraph from the BS 13 to the BS 14 depicted in FIG. 6.

[0099] The three paths depicted in FIG. 6 are the set of all possiblepaths from the BS 13 to the BS 14.

[0100] The vertex of each path is a part of base stations, which are thegraph vertexes. The first vertex of each path is the BS 13 and the lastvertex is the BS 14. The second vertex of the first path is the BS 12;adjacent to the first vertex being the BS 13. The second vertex of thethird path is the BS 15; adjacent to the first vertex being the BS 13.The third vertex of the third path is the BS 16; adjacent to the secondvertex of the third path being the BS 15.

[0101] The adjacent base stations are base stations for which theadjusted time difference of their signals has been obtained. Forinstance, the BS 15 and the BS 16 are adjacent, since the adjusted timedifference Δt_(16−>15,20) of their signals has been obtained.

[0102] The direction of each path is defined from the BS 13 to the BS14.

[0103] Six adjusted time differences have been obtained. Thus, thedigits from 1 to 6 number these adjusted time differences by thefollowing equation 5.

Δt ₁ =Δ{tilde over (t)} _(13−>12,17),

Δt ₂ =Δ{tilde over (t)} _(13−>14,17),

Δt ₃ =Δ{tilde over (t)} _(12−>14,17),

Δt ₄ =Δ{tilde over (t)} _(15−>13,18)

Δt ₅ =Δ{tilde over (t)} _(14−>16,19), and

Δt ₆ =Δ{tilde over (t)} _(16−>15,20).  Equation 5

[0104] The terms of the vector of the adjusted time differences and thevector of the adjusted time difference accuracies are here introduced.

[0105] The vector of the adjusted time differences Δ{right arrow over(t)} having the length of six, being number of the obtained adjustedtime differences, means a vector expressed by the following equation 6.$\begin{matrix}{{\Delta \quad \overset{\rightarrow}{t}} = {\begin{bmatrix}{\Delta \quad t_{1}} \\{\Delta \quad t_{2}} \\{\Delta \quad t_{3}} \\{\Delta \quad t_{4}} \\{\Delta \quad t_{5}} \\{\Delta \quad t_{6}}\end{bmatrix} = \begin{bmatrix}{\Delta {\overset{\sim}{t}}_{{13->12},17}} \\{\Delta {\overset{\sim}{t}}_{{13->14},17}} \\{\Delta {\overset{\sim}{t}}_{{12->14},17}} \\{\Delta {\overset{\sim}{t}}_{{15->13},18}} \\{\Delta {\overset{\sim}{t}}_{{14->16},19}} \\{\Delta {\overset{\sim}{t}}_{{16->15},20}}\end{bmatrix}}} & {{Equation}\quad 6}\end{matrix}$

[0106] In equation 6, p-th element Δt_(p) of the vector Δ{right arrowover (t)}, where p gets the values from 1 to 6, equals to the p-thadjusted time difference of the signals of the base stations.

[0107] The vector of the adjusted time difference accuracies {rightarrow over (σ)} having the length of six, being number of the obtainedadjusted time differences, refers to a vector expressed by the followingequation 7. $\begin{matrix}{\overset{\rightarrow}{\sigma} = {\begin{bmatrix}\sigma_{1} \\\sigma_{2} \\\sigma_{3} \\\sigma_{4} \\\sigma_{5} \\\sigma_{6}\end{bmatrix} = \begin{bmatrix}\sigma_{{13->12},17} \\\sigma_{{13->14},17} \\\sigma_{{12->14},17} \\\sigma_{{15->13},18} \\\sigma_{{14->16},19} \\\sigma_{{16->15},20}\end{bmatrix}}} & {{Equation}\quad 7}\end{matrix}$

[0108] In equation 7, p-th element σ_(p) of the vector {right arrow over(σ)}, where p gets the values from 1 to 6, equals to the accuracy of thep-th adjusted time difference of the base station signals.

[0109] The path vector enumerating the adjusted time differences of thebase station signals comprising this path is formed for each path fromthe formed set of all possible paths from the BS 13 to the BS 14.

[0110] The length of each path vector of six, being number of theadjusted time differences, is set.

[0111] The p-th element of the u-the path vector (where p takes thevalues within 1-6 and u takes the values from 1 to 3) is equal to 1 ifthe path has an arc corresponding to the p-th adjusted time differenceand the direction of the pass of the arc of the path complies with thedirection of the p-th adjusted time difference, where the direction ofthe p-th adjusted time difference of the i_(p)-th base station withrespect to the j_(p)-th base station is defined from the i_(p)-th basestation to the j_(p)-th base station; it is equal to −1 if the path hasan arc corresponding to the p-th adjusted time difference and thedirection of the pass of the arc of the path is reverse to the directionof the p-th adjusted time difference; otherwise it is equal 0.

[0112] The above described action results in three path vectors {rightarrow over (B)}¹, {right arrow over (B)}² and {right arrow over (B)}³corresponding to three paths from the BS 13 to the BS 14. These pathvectors equal to the following equation 8. $\begin{matrix}{{{\overset{\rightarrow}{B}}^{\quad_{1}} = \begin{bmatrix}1 \\0 \\1 \\0 \\0 \\0\end{bmatrix}},{{\overset{\rightarrow}{B}}^{\quad_{2}} = \begin{bmatrix}0 \\1 \\0 \\0 \\0 \\0\end{bmatrix}},{{\overset{\rightarrow}{B}}^{\quad_{3}} = \begin{bmatrix}0 \\0 \\0 \\{- 1} \\{- 1} \\{- 1}\end{bmatrix}}} & {{Equation}\quad 8}\end{matrix}$

[0113] The metric of the u-th path vector where u takes the values from1 to 3 is determined by the following equation 9. $\begin{matrix}{\sum\limits_{p = 1}^{6}\quad {{b_{p}^{u}}\sigma_{p}^{2}}} & {{Equation}\quad 9}\end{matrix}$

[0114] In equation 9, b_(p) ^(u) is p-th element of the u-th pathvector, and σ_(p) is p-th element of the vector {right arrow over (σ)}of the adjusted time difference accuracies of the base stations.

[0115] Thus, the metric of the first path vector {right arrow over (B)}¹is σ₁ ²+σ₃ ², the metric of the second path vector {right arrow over(B)}² is σ₂ ²; and the metric of the third path vector {right arrow over(B)}³ is σ₄ ²+σ₅ ²+σ₆ ².

[0116] A group of path vectors is selected for each pair of basestations from the set of all possible path vectors of this pair of basestations, wherein the selected group of the path vectors contains eachof the obtained adjusted time differences; wherein the number ofapplications of each obtained adjusted time difference of the selectedgroup of path vectors must not exceed the number of applications of thisadjusted time difference of any other group of path vectors obtainedfrom the set of all possible path vectors, wherein values of path vectormetrics of the selected group must not exceed values of path vectormetrics of any other path vector group obtained from the set of allpossible path vectors.

[0117] These three criteria of the path vector group selection will beexplained in greater details further.

[0118] The selected path vector group of this example complies with theset of all possible path vectors from the BS 13 to the BS 14.

[0119] Three estimates of the mutual time difference Δ{tilde over(t)}_(13−>14) of the signal of the BS 13 with respect to the signal ofthe BS 14 can be formed using the vector of the adjusted timedifferences Δ{right arrow over (t)} and three formed path vectors {rightarrow over (B)}¹, {right arrow over (B)}² and {right arrow over (B)}³.In this case, the r-th estimate of the mutual time difference Δ{tildeover (t)}_(13−>14) where r takes the values from 1 to 3 is given by thefollowing equation 10. $\begin{matrix}{\sum\limits_{p = 1}^{3}\quad {b_{p}^{r}\Delta \quad t_{p}}} & {{Equation}\quad 10}\end{matrix}$

[0120] In equation 10, b_(p) ^(r) is the p-th element of the r-th vectorof the path {right arrow over (B)}^(r), and Δt_(p) is the p-th adjustedtime difference (p-th element of the mutual time difference vectorΔ{right arrow over (t)}).

[0121] An error ε_(r) of the r-th mutual time difference estimateΔ{tilde over (t)}_(13−>14) with respect to the true value of the timedifference Δt_(13−>14) of the signal of the BS 13 with respect to thesignal of the BS 14 is expressed by the following equation 11.$\begin{matrix}{ɛ_{r} = {{\Delta \quad t_{13->14}} - {\sum\limits_{p = 1}^{3}\quad {b_{p}^{r}\Delta \quad t_{p}}}}} & {{Equation}\quad 11}\end{matrix}$

[0122] The correlation matrix between the estimate errors of the mutualtime difference Δ{tilde over (t)}_(13−>14) obtained by separate pathvectors is formed with the use of the vector of accuracies of theadjusted time differences of the signals of the base station {rightarrow over (σ)} and three generated path vectors {right arrow over(B)}¹, {right arrow over (B)}² and {right arrow over (B)}³, and thecorrelation matrix size is [3×3]. The correlation matrix element havingindex marks r₁ and r₂ (where r₁ and r₂ take the values from 1 to 3) isexpressed by the following equation 12. $\begin{matrix}{k_{r_{1},r_{2}} = {{K\left\lbrack {ɛ_{r_{1}},ɛ_{r_{2}}} \right\rbrack} = {\sum\limits_{p = 1}^{6}\quad {b_{p}^{r_{1}}b_{p}^{r_{2}}\sigma_{p}^{2}}}}} & {{Equation}\quad 12}\end{matrix}$

[0123] In equation 12, K [ε_(r) ₁ ,ε_(r) ₂ ] is the correlationcoefficient between the error ε_(r) ₁ of the r₁-th estimate of themutual time difference Δt_(13−>14) and the error ε_(r) ₂ of the r₂-thestimate of the mutual time difference Δt_(13−>14), b_(p) ^(r) ^(₁) isthe p-th element of the r₁-th path vector {right arrow over (B)}^(r)^(₁) , b_(p) ^(r) ^(₁) is the p-th element of the r₂-th path vector{right arrow over (B)}^(r) ^(₂) .

[0124] The correlation matrix {circumflex over (K)}_(13−>14) of thisexample is expressed by the following equation 13. $\begin{matrix}{{\hat{K}}_{13->14} = {\begin{bmatrix}{\sigma_{1}^{2} + \sigma_{3}^{2}} & 0 & 0 \\0 & \sigma_{2}^{2} & 0 \\0 & 0 & {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}}\end{bmatrix}.}} & {{Equation}\quad 13}\end{matrix}$

[0125] In general, the correlation matrix is non-diagonal.

[0126] The matrix Ŵ_(13−>14), being inverse to the obtained correlationmatrix {circumflex over (K)}_(13−>14) is formed. The matrix Ŵ_(13−>14)inverse to the formed correlation matrix {circumflex over (K)}_(13−>14)of the present example is expressed by the following equation 14.$\begin{matrix}{{\hat{W}}_{13->14} = {\left( {\hat{K}}_{13->14} \right)^{- 1} = {\begin{bmatrix}\frac{1}{\sigma_{1}^{2} + \sigma_{3}^{2}} & 0 & 0 \\0 & \frac{1}{\sigma_{2}^{2}} & 0 \\0 & 0 & \frac{1}{\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}}\end{bmatrix}.}}} & {{Equation}\quad 14}\end{matrix}$

[0127] With the use of the formed path vector group consisting of thepath vectors {right arrow over (B)}¹, {right arrow over (B)}² and {rightarrow over (B)}³ and the formed matrix Ŵ_(13−>14) inverse to thegenerated correlation matrix {circumflex over (K)}_(13−>14), six weightsof the adjusted time differences are produced, and the weight of thep-th adjusted time difference a_(p) (where p takes the values from 1 to6) may be calculated by the following equation 15. $\begin{matrix}{a_{p} = \frac{\sum\limits_{r_{1} = 1}^{3}\quad {\sum\limits_{r_{2} = 1}^{3}\quad {w_{r_{1},r_{2}}\left( {b_{p}^{r_{1}} + b_{p}^{r_{2}}} \right)}}}{2{\sum\limits_{r_{1} = 1}^{3}\quad {\sum\limits_{r_{2} = 1}^{3}\quad w_{r_{1},r_{2}}}}}} & {{Equation}\quad 15}\end{matrix}$

[0128] In equation 15, w_(r) ₁ _(, r) ₂ is an element of the formedmatrix Ŵ_(13−>14) with the indices r₁ and r₂, where r₁ and r₂ have thevalues from 1 to 3, b_(p) ^(r) ^(₁) is the p-th element of the r₁-thpath vector {right arrow over (B)}^(r) ^(₁) and b_(p) ^(r) ^(₂) is thep-th element of the r₂-th path vector {right arrow over (B)}^(r) ^(₂) .

[0129] The weights of the adjusted time differences of the presentexample equal to those shown in the following equation 16.$\begin{matrix}{{a_{1} = {a_{3} = \frac{\sigma_{2}^{2}\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)}{{\sigma_{2}^{2}\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)} + {\sigma_{2}^{2}\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)} + {\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)}}}},{a_{2} = \frac{\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)}{{\sigma_{2}^{2}\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)} + {\sigma_{2}^{2}\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)} + {\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)}}},{a_{4} = {a_{5} = {a_{6} = {- \frac{\sigma_{2}^{2}\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)}{{\sigma_{2}^{2}\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)} + {\sigma_{2}^{2}\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)} + {\left( {\sigma_{1}^{2} + \sigma_{3}^{2}} \right)\left( {\sigma_{4}^{2} + \sigma_{5}^{2} + \sigma_{6}^{2}} \right)}}}}}}} & {{Equation}\quad 16}\end{matrix}$

[0130] The mutual time difference Δ{tilde over (t)}_(13−>14) of thesignal of the BS 13 with respect to the signal of the BS 14 isdetermined by the following equation 17. $\begin{matrix}{{\Delta \quad {\overset{\sim}{t}}_{1314}} = {\sum\limits_{p = 1}^{6}\quad {a_{p}\Delta \quad t_{p}}}} & {{Equation}\quad 17}\end{matrix}$

[0131] That is, it is determined as the weighted sum of all adjustedtime differences Δt_(p) of the signals of the base stations, wherein theweights a_(p) of the adjusted time differences of the signals of thebase stations, where p has values from 1 to 6, are used as weights.

[0132] The radio communication cellular system comprising the basestation controllers, base stations, location measurement units and amobile user location center, which can practice the embodiment of thepresent invention as described above, will now be discussed, whereineach base station is controlled by one base station controller, eachlocation measurement unit receives signals of at least two basestations; and a signal of each base station is received by at least onelocation measurement unit.

[0133] The base stations, base station controllers and the mobile userlocation center can be embodied in a manner described in Patent WO#99/57826 titled “Method Of Synchronization Of A Base Station Network”,4 May 1998, Int. Cl. H 04 J 3/06, H 04 B 7/26, the contents of which isincorporated herein by reference.

[0134] Below is the brief description of units to include in the listedcomponents of the cellular radio communication network to embody themethod of mutual time difference determination of base station signalsin a cellular communication system in accordance with an embodiment ofthe present invention.

[0135] Each base station transmits a signal being its group signal.

[0136] Each location measurement unit measures sequentially the timedifference of signals of at least two base stations, which are signalsto be received by the location measurement unit, these time differencemeasurements are averaged, thus obtaining the averaged measured timedifference of signals of the base stations. The accuracy of the averagedmeasured time difference of signals of the base stations is determined.The averaged measured time differences and their accuracies aretransmitted from each location measurement unit to one of the basestations. The signals are received by the location measurement unit, andthen to the base station controller controlling the base station.

[0137] Referring to FIG. 7, the BS 31 should comprise at least areceiver 34 of a signal containing the averaged measured time differenceof signals of a base station pair and its accuracy (hereinafter referredto as receiver 34), a transmitter 35 of a signal containing the averagedmeasured time difference of signals of a base stations pair and itsaccuracy (hereinafter referred to as transmitter 35).

[0138] With reference to FIG. 8, the BS 31 executes the following steps.

[0139] In step S25, the receiver 34 receives a signal, containing theaveraged measured time difference of signals of a base station pair andits accuracy, transmitted from the LMU 30. In step S30, the transmitter35 transmits a signal containing the averaged measured time differenceof signals of a base station pair and its accuracy to the BSC 32.

[0140] Referring to FIG. 9, the BSC 32 should comprise at least areceiver 36 of a signal containing the averaged measured time differenceof signals of a base stations pair and its accuracy, a computing unit 37and a transmitter 38 of a signal containing the adjusted time differenceof signals of a base station pair and its accuracy.

[0141] As shown in FIG. 10, the following operations are performed usingthe BSC 32. In step 130, the receiver 36 receives a signal containingthe averaged measured time difference of signals of a base station pairand its accuracy transmitted by the BS 31. In step 135, the computingunit 37 subtracts the known value of the delay difference at the line ofsight signal propagation from the first BS and second BS of the pair tothe location measurement unit where this averaged measured timedifference has been obtained from the averaged measured time differencesignals of each pair of base stations, thus obtaining the adjusted timedifference of signals of this base stations pair. In step 140, thetransmitter 38 transmits a signal containing the adjusted timedifference of the base stations pair and its accuracy to the MULC 33.

[0142] Referring to FIG. 11, the MULC 33 comprises at least a receiver39 of a signal containing the adjusted time difference of signals of thebase stations pair and its accuracy (hereinafter referred to as thereceiver 39) and a computing unit 40.

[0143] The following steps are performed by the mobile user locationcenter as shown in FIG. 12. In step 240, the receiver 39 receives asignal containing the adjusted time difference of signals of the basestation pair and its accuracy transmitted by the BSC 32.

[0144] In step 245, the computing unit 40 forms a set of all possiblepaths from the first BS to the second BS for each pair of base stations.Said all possible paths refer to paths between base stations adjacent toa terminal which is an object of the measurement, and the paths betweenthe base stations adjacent to the terminal include non-line-of-sightmultipaths. Assume, the radio communication cellular system consists ofL base stations. All base stations are assumed to be numbered from 1through L.

[0145] Below is the explanation of the term “adjacent base station”. TheBS i is adjacent to the BS j, (wherein i and j takes the values from 1to L) if at least one adjusted time difference of a signal of the BS iis obtained with respect to a signal of the BS j. If p adjusted timedifferences of a signal of the BS i is obtained with respect to a signalof the BS j, these base stations are adjacent to each other p times.

[0146] The set of adjacent base stations is formed for each base stationin the following manner. Assume, the number of the base stationsadjacent to the BS i is Q_(i), where i takes the values from 1 to L. Theset of base stations adjacent to the BS i is defined by the followingequation 18.

D_(i)={d₁ ^(i), d₂ ^(i), . . . , d_(Q) _(i) ^(i)}  Equation 18

[0147] In equation 18, q_(i)-th element d_(q) _(i) ^(i) of the set

D_(i), where q_(i) takes the values from 1 to Q_(i), is the number ofthe q_(i)-th base station adjacent to the BS i. If any base station isan adjacent base station to the BS i p times, its number is included tothe

D_(i) set p times. L sets

D_(i) of the adjacent base stations are formed.

[0148] The set of all possible paths from the first BS to the second BSof the pair is formed for each pair of base stations in the followingmanner.

[0149] Assume, all paths from the BS i_(m) to the BS j_(m) have to bedetermined, where i_(m) and j_(m) have the values from 1 to L. This isperformed by the sequential selection of the following: all basestations adjacent to the BS i_(m), all base stations adjacent to thebase stations which are adjacent to the BS i_(m), all base stationadjacent to the base stations of adjacent base stations which areadjacent to the BS i_(m), etc.

[0150] The sequential selection is performed: the selection sequence tobe determined uniquely by sequences of the numbers of base station {d₀^(i), d₁ ^(i), . . . , d_(Q) _(i) ^(i)} of the formed sets

D_(i) of the adjacent base stations; each path to be updated with thosenumbers of base stations, which are not found previously in thesequential selection. In general, the lengths of the resulting paths aredifferent and the length of each path is less or equal to number of thebase stations L. Each path is a sequence of the numbers of the basestations in pass during the sequential selection. The first element ofthe sequence of base station numbers of each formed path is i_(m). Thelast element of the sequence of base station numbers of each formed pathcan be either j_(m) or not. Those formed paths which end at the BS j_(m)are kept, i.e. those formed paths with the last element of the sequenceof base station numbers of j_(m) are kept. Number of the paths kept isassumed to be U. They form a set of all possible paths from the BS i_(m)to the BS j_(m). They are numbered with the numbers from 1 to U.

[0151] In step 250, a path vector listing the adjusted time differencesof signals of base stations included into this path is formed for eachpath of each formed set; the metric of the said path vector isdetermined.

[0152] Assume, P adjusted time differences of the base station signalsare obtained. They are numbered by the numbers from 1 to P. The terms ofan adjusted time difference vector and an adjusted time differenceaccuracy vector are introduced.

[0153] The adjusted time difference vector Δ{right arrow over (t)} withthe length of P (number of the obtained adjusted time differences) isexpressed by the following equation 19. $\begin{matrix}{{\Delta \quad \overset{\rightarrow}{t}} = \begin{bmatrix}{\Delta \quad t_{1}} \\{\Delta \quad t_{2}} \\\vdots \\{\Delta \quad t_{P}}\end{bmatrix}} & {{Equation}\quad 19}\end{matrix}$

[0154] In equation 19, the p-th element Δt_(p) of the vector Δ{rightarrow over (t)} (where p takes the values from 1 to P) equals to thep-th adjusted time difference of the base station signals.

[0155] The vector of the adjusted time difference accuracies {rightarrow over (σ)} with the length of P (number of the obtained adjustedtime differences) is a vector expressed by the following equation 20.$\begin{matrix}{\quad {\overset{\rightarrow}{\sigma} = \begin{bmatrix}\sigma_{1} \\\sigma_{2} \\\vdots \\\sigma_{P}\end{bmatrix}}} & {{Equation}\quad 20}\end{matrix}$

[0156] In equation 20, the p-th element σ_(p) of the vector {right arrowover (σ)} (where p takes the values from 1 to P) equals to the accuracyof the p-th adjusted time difference of the base station signals.

[0157] The path vector enumerating the adjusted time differences of thesignals of the base stations comprised to this path is formed for eachpath of the generated set of all possible paths.

[0158] To illustrate, consider this operation for the set of allpossible paths from the BS i_(m) to the BS j_(m).

[0159] The length of each path vector is set to be P, number of theadjusted time differences.

[0160] The p-th element of the u-th path vector (where p takes thevalues from 1 to P and u takes the values from 1 to U) is equal to 1, ifthis path has an arc corresponding to the p-th adjusted time differenceand the direction of the pass of the arc of the path complies with thedirection of the p-th adjusted time difference; it is equal to −1, ifthe path has an arc corresponding to the p-th adjusted time differenceand the direction of the pass of the arc of the path is reverse to thedirection of the p-th adjusted time difference; and otherwise it isequal 0.

[0161] U vectors of the {right arrow over (B)}¹, . . . , {right arrowover (B)}^(U) paths from the BS i_(m) to the BS j_(m) are obtained. Theformed set of the vectors of all possible paths from the BS i_(m) to theBS j_(m) is generally redundant.

[0162] The metric of each formed path vector is determined. Forinstance, the metric of the u-th vector {right arrow over (B)}^(u) ofthe set of vectors of all possible paths from the BS i_(m) to the BSj_(m) is determined in the manner expressed by the following equation21. $\begin{matrix}{\sum\limits_{p = 1}^{P}\quad {{b_{p}^{u}}\sigma_{p}^{2}}} & {{Equation}\quad 21}\end{matrix}$

[0163] In equation 21, b_(p) ^(u) is the p-th element of the u-th vectorof the path {right arrow over (B)}^(u) from the BS i_(m) to the BSj_(m), and σ_(p) is the p-th element of the vector {right arrow over(σ)} of the adjusted time difference accuracies.

[0164] In step 255, a group of path vectors is selected for each pair ofbase stations from the set of all possible path vectors of this pair ofbase stations, wherein the selected group of the path vectors containseach of the obtained adjusted time differences; wherein number ofapplications of each obtained adjusted time difference of the selectedgroup of path vectors must not exceed the number of applications of thisadjusted time difference of any other group of path vectors obtainedfrom the set of all possible path vectors, wherein values of path vectormetrics of the selected group must not exceed values of path vectormetrics of any other path vector group obtained from the set of allpossible path vectors.

[0165] This operation is demonstrated by an example of the selection ofa group of the path vectors from the set of all possible path vectorsfrom the BS i_(m) to the BS j_(m).

[0166] The set of all possible path vectors from the BS i_(m) to the BSj_(m) is sorted in order of increasing path vector metric. The sortingresults in the obtaining of U vectors of the paths {right arrow over(B)}¹, . . . , {right arrow over (B)}^(U) where the vector of the path{right arrow over (B)}¹ has the minimum metric and the vector of thepath {right arrow over (B)}^(U) has the maximum metric.

[0167] The vector of the adjusted time differences {right arrow over(C)} of the {right arrow over (B)}^(u) path vector is defined. The{right arrow over (C)} vector length is P, and the p-th element of thevector {right arrow over (C)} equals to equation 22 below.$\begin{matrix}{c_{p} = {b_{p}^{u}}} & {{Equation}\quad 22}\end{matrix}$

[0168] That is to say, the element equals to the absolute value of thep-th element of the vector of the path {right arrow over (B)}^(u).

[0169] The adjusted time difference vector {right arrow over (C)} of the{right arrow over (B)}^(u) ^(₁) , {right arrow over (B)}^(u) ^(₂) , . .. path vectors is defined. The length of the vector {right arrow over(C)} is P, and the p-th element of the vector {right arrow over (C)} isdetermined by the manner below: c_(p)=0 if b_(p) ^(u) ^(₁) =0, b_(p)^(u) ^(₂) =0, . . . , otherwise c_(p)=1.

[0170] To select the path vector group from the set of all possible pathvectors from the BS i_(m) to the BS j_(m), {right arrow over (B)}², . .. , {right arrow over (B)}^(U) are tried to be removed sequentially fromthe path vector set at U−1 steps. In this case the vector of the path{right arrow over (B)}^(u) is attempted to be removed at the u-th step(where u takes the values from 2 to U) in the following manner:

[0171] the vector {right arrow over (C)} is formed as a vector of theadjusted time differences of the vectors of the paths {right arrow over(B)}^(u−1), {right arrow over (B)}^(u−2), . . . , {right arrow over(B)}¹,

[0172] if b_(p) ^(u)=0 for all p (where p takes the values from 1 to p)for which c_(p)=0, the vector of the path {right arrow over (B)}^(u) canbe removed,

[0173] otherwise, it cannot be removed.

[0174] Assume, we have R path vectors from the BS i_(m) to the BS j_(m).They form the selected group of the path vectors {right arrow over(B)}¹, . . . , {right arrow over (B)}^(R) from the BS i_(m) to the BSj_(m) which will be further used.

[0175] In step 260, weights of the adjusted time differences of signalsof base stations are formed for each pair of base stations using theselected group of path vectors of these base stations and the obtainedaccuracies of the adjusted time differences of signals of the basestations. That is, for said all paths, weights are provided according toerrors of the measured mutual time differences. This situation isexplained by the forming of the weights of the adjusted time differenceof BS i_(m) and BS j_(m) signals.

[0176] The correlation matrix of errors of mutual time difference of thesignal of the BS i_(m) with respect to the signal of the BS j_(m)obtained by separate path vectors is formed using the accuracy vector{right arrow over (σ)} of the adjusted time differences of the basestation signals and the selected path vectors group {right arrow over(B)}¹, . . . , {right arrow over (B)}^(R).

[0177] The correlation matrix size is [R×R]. The correlation matrixelement with the indices r₁ and r₂ (wherein r₁ and r₂ taking the valueswithin 1−R) is expressed by the following equation 23. $\begin{matrix}{k_{r_{1},r_{2}} = {{K\left\lbrack {ɛ_{r_{1}},ɛ_{r_{2}}} \right\rbrack} = {\sum\limits_{p = 1}^{P}\quad {b_{p}^{r_{1}}b_{p}^{r_{2}}\sigma_{p}^{2}}}}} & {{Equation}\quad 23}\end{matrix}$

[0178] In equation 23, K [ε_(r) ₁ ,ε_(r) ₂ ] is the correlationcoefficient between the r_(i)-th and r₂-th errors of the estimate of themutual time difference of the signal of the BS i_(m) with respect to thesignal of the BS j_(m), b_(p) ^(r) ^(₁) is the p-th element of the r₁-thpath vector {right arrow over (B)}^(r) ^(₁) , and b_(p) ^(r) ^(₂) is thep-th element of the r₂-th path vector {right arrow over (B)}^(r) ^(₂) .

[0179] The matrix Ŵ, inverse to the formed correlation matrix{circumflex over (K)} is produced.

[0180] P weights of the adjusted time differences are generated usingthe selected group of the path vectors consisting of the path vectors{right arrow over (B)}¹, . . . , {right arrow over (B)}^(R) and usingthe generated matrix Ŵ, inverse to the formed correlation matrix{circumflex over (K)}, the weight of the p-th adjusted time differencea_(p) (wherein p taking the values from 1 to P) being calculated by thefollowing equation 24. $\begin{matrix}{a_{p} = \frac{\sum\limits_{r_{1} = 1}^{R}\quad {\sum\limits_{r_{2} = 1}^{R}\quad {w_{r_{1},r_{2}}\left( {b_{p}^{r_{1}} + b_{p}^{r_{2}}} \right)}}}{2{\sum\limits_{r_{1} = 1}^{R}\quad {\sum\limits_{r_{2} = 1}^{R}\quad w_{r_{1},r_{2}}}}}} & {{Equation}\quad 24}\end{matrix}$

[0181] In equation 24, w_(r) ₁ _(,r) ₂ is an element of the formedmatrix Ŵ with the indices r₁ and r₂, where r₁ and r₂ take the valuesfrom 1 to R, b_(p) ^(r) ^(₁) is the p-th element of the r₁-th pathvector {right arrow over (B)}^(r) ^(₁) , and b_(p) ^(r) ^(₂) is the p-thelement of the r₂-th path vector {right arrow over (B)}^(r) ^(₂) .

[0182] In step 265, a mutual time difference of signals of each pair ofbase stations is determined as a weighted sum of all adjusted timedifferences of signals of base stations, wherein the weights of theadjusted time difference of signals of the base stations formed for thepair of base stations are used as weights.

[0183] For example, the mutual time difference Δ{tilde over (t)}_(i)_(m) _(−>j) _(m) of the signal of the BS i_(m) with respect to thesignal of the BS j_(m) is determined by the following equation 25.$\begin{matrix}{{\Delta \quad {\overset{\sim}{t}}_{i_{m}j_{m}}} = {\sum\limits_{p = 1}^{P}\quad {a_{p}\Delta \quad t_{p}}}} & {{Equation}\quad 25}\end{matrix}$

[0184] That is to say, it is determined as the weighted sum of alladjusted time differences Δt_(p) of the base station signals wherein theweights a_(p) of the adjusted time differences of the base stationsignals formed for the BS i_(m) and BS j_(m), where p takes the valueswithin 1 . . . P, are used as weights.

[0185] Hereinafter, the joint operation of the cellular systemcomponents for the implementation of the claimed invention will bedescribed with reference to FIG. 13. FIG. 13 shows the LMU 30, BS 31,BSC 32 and the MULC 33.

[0186] In step 310, the LMU 30 sequentially measures the time differenceof signals of at least two received base stations. In step 315, the LMU30 averages these time difference measurements, thereby obtaining theaveraged measured time difference of signals of these base stations. Instep 320, the LMU 30 determines the accuracy of the averaged measuredtime difference of signals of these base stations. In step 325, the LMUtransmits the first signal containing the averaged measured timedifference of the signals of the base stations pair and its accuracy tothe BS 31. In step 330, the BS 31 transmits the second signal containingthe averaged measured time difference of the signals of the basestations pair and its accuracy to the BSC 32.

[0187] In step 335, in the BSC 32, the known value of the delaydifference at the line of sight signal propagation from the first BS andsecond BS of each pair of the base stations to the location measurementunit, where this averaged time difference is obtained, is subtractedfrom the averaged measured time difference of this pair of the basestations, thus obtaining the adjusted time difference of the signals ofthis base stations pair.

[0188] In step 340, the BSC 32 transmits the third signal containing theadjusted time difference of each base stations pair and its accuracy tothe MULC 33.

[0189] In step 345, the MULC 33 forms a set of all possible paths fromthe first BS to the second BS of the pair for each base stations pair.Said all possible paths refer to paths between base stations adjacent toa terminal which is an object of the measurement, and the paths betweenthe base stations adjacent to the terminal include non-line-of-sightmultipaths.

[0190] In step 350, the MULC 33 forms a path vector enumerating theadjusted time differences of the base station signals included into thispath and determines its metric for each path of each generated set.

[0191] In step 355, the MULC 33 selects a group of path vectors for eachpair of base stations from the set of all possible path vectors of thispair of base stations, wherein the selected group of the path vectorscontains each of the obtained adjusted time differences; wherein numberof applications of each obtained adjusted time difference of theselected group of path vectors must not exceed number of applications ofthis adjusted time difference of any other group of path vectorsobtained from the set of all possible path vectors, wherein values ofpath vector metrics of the selected group must not exceed values of pathvector metrics of any other path vector group obtained from the set ofall possible path vectors.

[0192] In step 360, the MULC 33 forms weights of the adjusted timedifferences of signals of base stations for each pair of base stationsusing the selected group of path vectors of these base stations and theobtained accuracies of the adjusted time differences of signals of thebase stations. For said all paths, weights are provided according toerrors of the measured mutual time differences.

[0193] In step 365, the MULC 33 determines a mutual time difference ofsignals of each pair of base stations as a weighted sum of all adjustedtime differences of signals of base stations, wherein the weights of theadjusted time difference of signals of the base stations formed for thepair of base stations are used as weights.

[0194] The claimed method of mutual time difference determination ofbase station signals in a cellular communication system has a numberadvantages compared to prior inventions known to those skilled in theart. First, the claimed method enables the determination of the mutualtime difference of signals of any pair of base stations regardless ofthe availability of the direct measurement of the adjusted timedifference of signals of this pair of base stations. Secondly, theclaimed method enables improved accuracy in determining the mutual timedifferences of base station signals. These advantages are achieved dueto joint statistical processing of all adjusted time differences of basestation signals.

[0195] While the invention has been shown and described with referenceto certain embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A method for determining mutual time differencebetween base station signals in an asynchronous code divisionmultiplexing access (CDMA) system, the method comprising the steps of:(a) measuring mutual time difference of signals transmitted between atleast two base stations; (b) determining all possible paths between saidat least two base stations; and (c) providing weights to the measuredmutual time difference for said all possible paths.
 2. The method asclaimed in claim 1, wherein each of the signals transmitted between saidat least two base stations is transmitted through a common pilotchannel.
 3. The method as claimed in claim 1, wherein step (a) comprisesthe substeps of: sequentially measuring mutual time difference of thesignals when received; and averaging measurements of the mutual timedifference, thereby obtaining an average of the measured time differencefor the signals.
 4. The method as claimed in claim 3, wherein step (a)further comprises a substep of determining an accuracy of the averagedmeasured time difference by means of a signal to noise ratio, so as tomeasure an error between the measured mutual time difference and theaverage of the measured mutual time difference.
 5. The method as claimedin claim 3, wherein the average of the measured mutual time differenceis compensated by subtracting a difference of delays at line of sightsignal from the average of the measured mutual time difference, saidline of sight signal propagating between said at least two basestations.
 6. The method as claimed in claim 5, wherein the difference ofdelays at line of sight signal is obtained by equations,$\tau_{i->k} = {\frac{\sqrt{\left( {x_{i} - x_{k}} \right)^{2} + \left( {y_{i} - y_{k}} \right)^{2} + \left( {z_{i} - z_{k}} \right)^{2}}}{c}\quad {and}}$${\tau_{j->k} = \frac{\sqrt{\left( {x_{j} - x_{k}} \right)^{2} + \left( {y_{j} - y_{k}} \right)^{2} + \left( {z_{j} - z_{k}} \right)^{2}}}{c}},$

in which c is the light speed, a first base station coordinates withx_(i), y_(i), z_(i), a second base station coordinates with x_(j),y_(j), z_(j), and a location measurement unit coordinates with x_(k),y_(k), z_(k).
 7. The method as claimed in claim 1, wherein step (a)comprises the substeps of: (1) receiving information about adjusted timedifference and the accuracy; (2) forming a set of all possible pathsbetween said at least two base stations; (3) for each path of eachformed set, forming a path vector listing the adjusted time differencesof signals of the base station included in this path and determining ametric of the path vector; (4) selecting a group of path vectors foreach pair of base stations from the set of all possible path vectors ofthis pair of base stations, wherein the selected group of the pathvectors contains each of the obtained adjusted time differences; (5)forming weights of the adjusted time differences of signals of basestations for each pair of base stations using the selected group of pathvectors of these base stations and obtained accuracies of the adjustedtime differences of signals of the base stations; and (6) determining amutual time difference of signals of each pair of base stations as aweighted sum of all adjusted time differences of signals of basestations, wherein the weights of the adjusted time difference of signalsof the base stations formed for the pair of base stations are used asweights.
 8. The method as claimed in claim 1, wherein the measuredmutual time difference of signals and its accuracy are transmittedthrough one of said at least two base stations to a base stationcontroller.
 9. The method as claimed in claim 1, wherein the measuredmutual time difference of signals and its accuracy are transmitted fromthe base station controller to a mobile user location center forcalculating the mutual time difference of the signals.
 10. The method asclaimed in claim 7, wherein, in step (4), a number of applications ofeach obtained adjusted time difference of the selected group of pathvectors does not exceed a number of applications of this adjusted timedifference of any other group of path vectors obtained from the set ofall possible path vectors, and values of path vector metrics of theselected group do not exceed values of path vector metrics of any otherpath vector group obtained from the set of all possible path vectors.11. The method as claimed in claim 1, wherein said all possible pathsrefer to paths between base stations adjacent to a terminal which is anobject of the measurement.
 12. The method as claimed in claim 11,wherein the paths between the base stations adjacent to the terminalinclude non-line-of-sight multipaths.
 13. The method as claimed in claim1, wherein, for said all possible paths, the weights are providedaccording to errors of the measured mutual time differences.
 14. Anapparatus for determining mutual time difference between base stationsignals in an asynchronous code division multiplexing access (CDMA)system, the apparatus comprising: a location measurement unit formeasuring mutual time difference of signals transmitted between at leasttwo base stations; a mobile user location center for receiving themutual time difference of the signals measured by the locationmeasurement unit, determining all possible paths between said at leasttwo base stations, and providing weights to the measured mutual timedifference for said all possible paths.
 15. The apparatus as claimed inclaim 11, wherein each of the signals transmitted between said at leasttwo base stations is transmitted through a common pilot channel.
 16. Theapparatus as claimed in claim 11, wherein the location measurement unitsequentially measures mutual time difference of the signals whenreceived, and averages measurements of the mutual time difference,thereby obtaining an average of the measured time difference for thesignals.
 17. The apparatus as claimed in claim 13, wherein the locationmeasurement unit determines an accuracy of the averaged measured timedifference by means of a signal to noise ratio, so as to measure anerror between the measured mutual time difference and the average of themeasured mutual time difference.
 18. The apparatus as claimed in claim13, wherein the location measurement unit compensates the average of themeasured mutual time difference by subtracting a difference of delays atline of sight signal from the average of the measured mutual timedifference, said line of sight signal propagating between said at leasttwo base stations.
 19. The apparatus as claimed in claim 15, wherein thelocation measurement unit calculates the difference of delays at line ofsight signal by means of equations,$\tau_{i->k} = {\frac{\sqrt{\left( {x_{i} - x_{k}} \right)^{2} + \left( {y_{i} - y_{k}} \right)^{2} + \left( {z_{i} - z_{k}} \right)^{2}}}{c}\quad {and}}$${\tau_{j->k} = \frac{\sqrt{\left( {x_{j} - x_{k}} \right)^{2} + \left( {y_{j} - y_{k}} \right)^{2} + \left( {z_{j} - z_{k}} \right)^{2}}}{c}},$

in which c is the light speed, a first base station coordinates withx_(i), y_(i), z_(i), a second base station coordinates with x_(j),y_(j), z_(j), and a location measurement unit coordinates with x_(k),y_(k), z_(k).
 20. The apparatus as claimed in claim 11, wherein themobile user location center performs: receiving information aboutadjusted time difference and the accuracy; forming a set of all possiblepaths between said at least two base stations; for each path of eachformed set, forming a path vector listing the adjusted time differencesof signals of the base station included in this path and determining ametric of the path vector; selecting a group of path vectors for eachpair of base stations from the set of all possible path vectors of thispair of base stations, wherein the selected group of the path vectorscontains each of the obtained adjusted time differences; forming weightsof the adjusted time differences of signals of base stations for eachpair of base stations using the selected group of path vectors of thesebase stations and obtained accuracies of the adjusted time differencesof signals of the base stations; and determining a mutual timedifference of signals of each pair of base stations as a weighted sum ofall adjusted time differences of signals of base stations, wherein theweights of the adjusted time difference of signals of the base stationsformed for the pair of base stations are used as weights.
 21. Theapparatus as claimed in claim 11, wherein the location measurement unittransmits the measured mutual time difference of signals and itsaccuracy through one of said at least two base stations to a basestation controller.
 22. The apparatus as claimed in claim 18, whereinthe mobile user location center receives the measured mutual timedifference of signals and its accuracy from the base station controller.23. The apparatus as claimed in claim 17, wherein in selecting a groupof path vectors, the mobile user location center prevents a number ofapplications of each obtained adjusted time difference of the selectedgroup of path vectors from exceeding a number of applications of thisadjusted time difference of any other group of path vectors obtainedfrom the set of all possible path vectors, and prevents values of pathvector metrics of the selected group from exceeding values of pathvector metrics of any other path vector group obtained from the set ofall possible path vectors.
 24. The apparatus as claimed in claim 14,wherein said all possible paths refer to paths between base stationsadjacent to a terminal which is an object of the measurement.
 25. Theapparatus as claimed in claim 24, wherein the paths between the basestations adjacent to the terminal include non-line-of-sight multipaths.26. The apparatus as claimed in claim 14, wherein, for said all possiblepaths, the weights are provided according to errors of the measuredmutual time differences.